Various publications, including patents, published applications, technical articles and scholarly articles are cited throughout the specification. Each of these cited publications is incorporated by reference herein, in its entirety. Full citations for publications not cited fully within the specification are set forth at the end of the specification.
The efficacy of the complement-mediated immune response relies on a delicate balance between activation and regulation. Although continuous generation of the strong opsonin complement fragment C3b by the alternative pathway (AP) allows rapid reaction to foreign or abnormal cells, its indiscriminate deposition may potentially cause host tissue damage. Host cells are therefore protected by proteins of the regulator of complement activation (RCA) family, which either impair the generation of new C3b by accelerating the decay of the C3 convertases or act as cofactor for factor 1 (F1) in degrading existing C3b. In addition to cell surface-bound RCA proteins such as decay accelerating factor (DAF (A000571); also called CD55), membrane cofactor protein (MCP (A000568); also called CD46) and complement receptor 1 (CR1; also called CD35), the soluble and highly abundant regulator factor H (FH) offers an additional layer of protection, as it controls the steady-state alternative pathway activation in circulation and on surfaces to which it specifically binds.
Factor H, the second most abundant complement protein in plasma, is the primary regulator of the AP. It is a large (155 kDa), soluble glycoprotein that circulates in human plasma. It has an elongated structure consisting of 20 homologous short consensus repeats (SCR, also referred to as complement control protein (CCP) domains or modules), each comprising approximately 60 residues that are common to all RCA proteins (Schmidt C Q et al., 2008, Clin Exp Immunol 151: 14-24), held together by four conserved cysteine residues. Whereas the complement regulatory functions are concentrated to the N-terminus (SCR 1-4) of factor H, two distinct regions (SCR 6-8, SCR 19-20) define the recognition of self-surfaces via binding to polyanion patches (e.g., glycosaminoglycans; GAG) on host cells. Factor H regulates the AP by inhibiting the formation of the AP C3 convertase and accelerating its dissociation, or by acting as cofactor for the degradation of C3b by factor I (Wu, J et al., 2009, Nat Immunol 10:728-733; Schmidt, C et al., 2008, J Immunol 181:2610-2619; Schmidt, C et al., 2008, Clin Exp Immunol 151:14-24; Pickering, M C et al., 2008, Clin Exp Immunol 151:210-230; Jozsi, M, & P F Zipfel, 2008, Trends Immunol 29:380-387; Ross, G D et al., 1982, J Immunol 129:2051-2060).
Though a soluble protein, FH may be recruited to host membranes by recognizing and binding self-components, such as glycosaminoglycans, and thereby may prevent the opsonization of host tissue with low surface expression of RCA. The importance of FH in maintaining a well-balanced immune response is reflected by the increasing number of diseases found to have strong association with mutations and polymorphisms in the gene encoding FH, as found in age-related macular degeneration (AMD), atypical hemolytic uremic syndrome (aHUS) and membrano-proliferative glomerulonephritis type II (MPGN-II) (Meri, S, 2007, Ann NY Acad Sci 1109: 93-105; de Cordoba, S R & de Jorge, E G, 2008, Clin Exp Immunol 151: 1-13). Because of their vital function in immune modulation, therapeutic targeting of FH and other RCA proteins has been considered important for the treatment of diseases associated with abnormal or loss of complement control.
In accordance with a current model of FH engagement of C3b on a self-surface, concurrent recognition of C3b-opsonins and host-surface markers enable FH to control complement activation effectively on self, but not on foreign surfaces. This specificity depends on the simultaneous binding of FH19-20 to the thioester domain (TED or C3d domain) of C3b, and to polyanions. Proteolytic cleavage within the complement C1r/C1s, Uegf, Bmp1 (CUB) domain of C3b inactivates C3b and yields iC3b (inactive C3b). With proteolysis of CUB, C3b loses one interaction patch for the N-terminal FH domains (Wu J et al., 2009 Nat Immunol 10: 728-733; Schmidt C Q et al., 2008, J Immunol 181: 2610-2619), but iC3b still contains TED, the interaction partner of C-terminal domains 19-20 of FH. Even so, FH no longer binds readily to iC3b under physiological conditions, irrespective of iC3b being positioned on a cell surface (Ross G D et al., 1983, J Exp Med 158: 334-352) or directly on a biosensor (Alcorlo M et al., 2011, Proc Natl Acad Sci USA 108: 13236-13240). In contrast, FH19-20 binds indiscriminately well to the TED of C3b, C3dg and C3d (Morgan H P et al., 2011, Nat Struct Mol Biol 18, 463-470).
A role of FH in abatement of oxidative stress was established recently. Malondialdehyde (MDA) is a common lipid peroxidation product that accumulates in many pathological processes, including AMD. FH has been identified as a major MDA-binding protein that can block both the uptake of MDA-modified proteins by macrophages and MDA-induced proinflammatory effects in vivo in mice (Weismann D et al., 2011, Nature 478: 76-81). SCRs 7 and 20 were demonstrated to mediate the binding of FH to MDA (Id.).
The AP has been shown to play a particularly important role in preclinical disease models and by studies of human diseases, where mutations or dysfunctional polymorphisms that promote activation of the AP are highly associated with diseases such as aHUS, dense deposit disease, AMD and paroxysomal nocturnal hemoglobinuria (PNH) (see Holers, V M, 2008, Immunol Rev 223: 300-316). Diseases in which a primary pathogenic role for the alternative pathway has been shown or postulated in or from preclinical studies in animals include rheumatoid arthritis (RA), systemic lupus erythematosus (SLE), several autoimmune and autoinflammatory kidney diseases, autoimmune myocarditis, multiple sclerosis, traumatic brain and spinal cord injury, intestinal and renal ischemia-reperfusion (IR) injury, spontaneous and recurrent pregnancy loss, anti-phospholipid syndrome (APS), asthma, anti-nuclear cytoplasmic antigen-associated pauci-immune vasculitis (Wegener's syndrome), non-lupus autoimmune skin diseases such as pemphigus, bullous pemphigoid, and epidermolysis bullosa, post-traumatic shock and atherosclerosis (Holers, 2008, supra).
A milestone in complement intervention strategies was marked by the approval of the humanized monoclonal antibody Eculizumab for the treatment of the orphan diseases PNH and aHUS. Both diseases are characterized by malfunctioning of complement regulators and result in insufficient control of the AP. Eculizumab binds C5 and inhibits its activation into C5a, a potent anaphylatoxin, and C5b, the initiator of the terminal pathway, and consequently inhibits inflammatory signaling and cell lysis by MAC. However, use of Eculizumab can be disadvantageous; for instance, its use increases susceptibility to infections. Additionally, Eculizumab treatment costs are extremely high and an appreciable proportion of PNH patients do not respond to Eculizumab treatment. These disadvantages have boosted the preclinical development of the chimeric fusion protein TT30, which intervenes earlier in the complement cascade at C3-level, is AP-specific and targets to sites of C3-inactivation products predominantly found on host surfaces (Fridkis-Hareli M, et al., 2011, Blood 118: 4705-4713).
From the foregoing discussion, it is clear that improved regulators of the AP of complement activation are needed. The present invention satisfies that need.